Many previously known polyimide and polyamide-imide resins suffer from their inability to be processed at elevated temperatures. These resins are often characterized by their insolubility and infusibility properties which make them practically impossible to process with conventional plastics process equipment. As a consequence, the resins are usually handled in the form of their precursor polyamide acids. The precursor materials, while suitable for the preparation of thin films prepared from solution, cannot be handled by autoclaving and press molding techniques without great difficulty. The polyamide acid forms of both the polyimide and polyamide-imide resins undergo cyclization to the imide form during processing, giving rise to poorly consolidated and void filled parts of low mechanical strength.
It has now been found that, by the addition of certain reactive Schiff base compounds to polyimide and polyamide-imide prepolymers or their corresponding polyamide acids containing unsaturated end groups, or by the addition of an N,N'-bis-imide compound to polyimide or polyamide-imide prepolymers or their corresponding polyamide acids containing a Schiff base end group, it is possible to process these materials using conventional plastic processing techniques such as autoclave and vacuum bag laminating, compression and injecting molding. These additives while lowering the softening temperature of these prepolymers, also chain-extend and cross-link the base resin to give useful objects which possess good mechanical and thermooxidative properties as well as low void content.